This invention relates to a multi-detector intensity interferometer method and apparatus for receiving incoherent radiation from a body and processing such radiation to determine information, such as the location of the body relative to a reference location, therefrom.
It has been recognized that incoherent radiation emanating from a body contains a spectrum of frequencies with random phase changes. Such radiation has been previously analyzed for selected puposes, but is not known to have been utilized to locate the three-dimensional coordinates of the body relative to a reference coordinate system.
An early device for analyzing such radiation is disclosed in Nature, Volume 170, page 1061, published in 1952, in which an intensity interferometer is used for the limited purpose of measuring the angular diameter of discreet radio sources. This device employed two radiation intensity detectors which produced outputs that were multiplied and the result integrated. The signals received at the two detectors were delayed relative to one another by varying the separation of the detectors.
A later apparatus was suggested by Gamo in an article entitled "Triple Correlator of Photoelectric Fluctuations as a Spectroscopic Tool" published in 1963 in the Journal of Applied Physics, Volume 34, No. 4. In this device, three signals were processed for use in analyzing asymmetrical spectrum profiles of laser beams. More specifically, a collimated beam of laser light was divided by two beam splitters into three signals which were then detected by three respective photodetectors. Two of the detected signals were then time-delayed and the signals were multiplied together, integrated and recorded for processing. Sato, et al, expanded this work in an article published in 1978 in Applied Optics, Volume 17, at page 2047. More specifically, in Sato, quasi-incoherent light was obtained by passing light from an he-ne laser through a rotating ground glass diffuser. The defracted light was then passed through a lens, a half mirror, a prism and detected. By measuring the magnitude and phase of the correlation functions, Sato reconstructed the image of a two-dimensional object from the detected signals.
Also, as exemplified by U.S. Pat. No. 3,346,862 of Raudsep, two radiation detectors have previously been used to receive signals from a radiation source. The received signals are processed and utilized to determine the angular position of the source with reference to the perpendicular bisector of a line containing the two detectors. In Raudsep, the two received signals are understood to have been passed through a weighting filter as part of the signal processing. This filter is designed to modify the received signals so that, after they are time-delayed and multiplied together, side peaks in an auto correlation function associated with the signals are minimized. Raudsep does incidentally mention that his receivers can be antennas or any other type of detectors. Also, without explaining how, Raudsep also mentions the use of triangulation techniques and the combining of outputs of pairs of detectors to obtain range information, in addition to information on the angular position of the source. Thus, Raudsep is understood to disclose an apparatus which analyzes radiation to determine limited range and bearing information concerning a source of radiation.
Another device for determining the range and bearing of a signal source is disclosed in U.S. Pat. No. 3,947,803 of Brown. In Brown, three spaced-apart amplitude sensitive hydrophones receive signals from a source. These signals are compared two at a time in respective circuits. Each such circuit includes a time-delay for delaying one of the two compared signals, a multiplier for multiplying the time-delayed signals with the non-time-delayed signals, and an integrator for integrating the multiplied output signal. A computer is then utilized to determine the range and bearing of the source based upon the results of the comparison. However, Brown is understood to process amplitude representations of the received signals and not intensity representations thereof. Furthermore, Brown integrates the product of only two signals at a time thereby limiting the information available from the signals.
A further device similar to Brown, is disclosed in U.S. Pat. No. 3,249,911 of Gustafsson, in which three amplitude detectors are positioned in a plane. In Gustafsson, the product of time-delayed respective first and second signals from two of the detectors is obtained. This product signal is then filtered, as by a low pass filter or integrator, to produce an output which is maximum if the two detected signals are coherent. In addition, the second signal and a third signal from the remaining detector are also respectively time-delayed, multiplied, and filtered to produce another output which is maximum if these latter two signals are coherent. Furthermore, two signals consisting of the product of the first and second signals and the product of the second and third signals are multiplied and filtered to produce a third filtered output signal. This third filtered output signal, according to Gustafsson, is non-zero only if the first, second and third detected signals are from the same signal source. Thus, according to Gustafsson, his device is capable of determining the position of a source in a two-dimensional plane. In addition, he mentions the possible use of more than three detector stations for over-determination of the two-dimensional location. However, Gustafsson is also understood to process amplitudes of detected signals and not intensities thereof. Furthermore, Gustafsson does not suggest determination of the three-dimensional corrdinates of a source of coherent radiation. For that matter, Gustafsson, as understood, is incapable of determining the three-dimensional location of a source. That is, a source can be at any point on the line of intersection of two hyperbolas, and not necessarily in the plane of the Gustafsson detectors, and Gustafsson cannot determine the location of the source along this line.
As an example of still another prior device, consider U.S. Pat. No. 3,430,243 of Evans. In Evans, a four detector device is disclosed for determining the distance or range between objects and the relative bearing and elevation angles therebetween. More specifically, Evans illustrates three detectors on an aircraft, positioned in an equilateral triangle, together with a fourth detector disposed in a line perpendicular to the plane containing the three detectors and which intersects one of the three detectors. Evans employs ratio determining circuits for comparing the ratio of the intensities of detected signals and circuitry for utilizing these ratios to determine the desired range and bearing information. Thus, Evans operates on an entirely different principle for determining limited information concerning the location of a signal source.
Therefore, a need exists for an apparatus and method for analyzing the intensity of incoherent radiation received from a body for the purpose of more accurately determining the location of the body relative to a reference location. Also, a need exists for an apparatus and method for more effectively processing such received radiation for a variety of purposes.